This invention relates to injection molding and, more particularly, to a method of predicting optimal injection molding cycle time.
Injection molding of parts made of thermoplastic material generally has three main phases: first, injection of material into the mold; second, packing and cooling of the material in the mold in forming the desired part, and third, ejection of the molded part from the mold. The molding cycle time is commonly referred to as the duration of time from the start of the injection phase to initiation of the ejection phase.
Ability to accurately predict molding cycle time is of paramount importance in injection molding, since it relates directly to the production rate and part quality. When cycle time exceeds the desired range, the production rate will be compromised. In some instances, due to excessively increased friction force, the part may exhibit brittle failure during ejection. Another possible situation is that the ejector pins can be damaged because the friction force exceeds the maximum ejection force that the machine can provide through the ejectors. If the part is ejected too early, only a thin layer of polymer is solidified and ejection may cause the part to be deformed permanently, which generally leads to surface defects. For most injection molders, cycle time is estimated through molding trials, which are very costly and time consuming. In testing a new material, it is even more difficult to determine a proper cycle time range due to lack of knowledge of the material behavior.
One prior art approach (used in research reported by: Yu et al. in Polymer Eng. and Sci., Vol. 32, pp 191, 1992; Dowler et al. in Plastics Engineering, June, 1997, pp 29; and Yang et al. in Polymer Technology, Vol. 15, pp 289, 1996) to predicting cycle time is based on a simple thermal analysis across the thickness of the part. Heat Deflection Temperature, known as HDT, is used as the ejection criterion, and the cooling period in which the part is cooled to the ejection temperature is considered as the cycle time. The concept implied in this prior art approach is that the part reaches maximum stiffness when it is completely solidified, and it is presumed that no damage will occur if the part is ejected thereafter. Such approach provides good representation of the part stiffness increase as cooling takes place and it is generally accepted as a quick estimate for cycle time in pre-design phases.
This prior art approach has several drawbacks. First, since the part has non-uniform temperature through its thickness, it is nontrivial to define a cycle time based on a single temperature such as HDT. Either the maximum temperature or averaged temperature might be compared with the ejection temperature. For semi-crystalline materials, it is well known that HDT is not suited to represent the polymer transition from the liquid to the solid phase. Using HDT as the ejection criterion becomes ambiguous and yields unrealistic prediction of cycle time. Second, the most important requirement for determining molding cycle time is that the part be ejected without damage, e.g., no permanent deformation such as visible marks, part brittle failure, etc. This prior art approach considers only the cooling aspect of the injection molding process and therefore cannot account for the processing conditions which have significant effects on stress generated by ejection, as well as on cycle time. For instance, the packing pressure plays an important role in determining optimal cycle time. Effective packing will facilitate the ejection in reducing friction force between plastics and steel molds and potentially reduce the cycle time. Third, this prior art approach always predicts cycle time regardless of the ejector pin layout and part geometry. Poor design of ejector pin sizes and locations can often lead to high stress in certain areas of the part that exceed the polymer yield stress. Therefore, permanent deformation such as pin push marks and part failure can occur when ejection takes place even if the part is cooled to the ejection temperature.
Another prior art approach (employed in research reported by Lai et al. in ANTEC""94, SPE, pp 733, 1994) uses a stiffness criterion instead of ejection temperature. This prior art approach suggests that the part can be ejected even if it is only partially solidified. By doing so, the cycle time is reduced. Since the part stiffness, which is solely dependent on temperature, does not relate to processing conditions, part design or pin layout, the stiffness approach has the same drawbacks as the ejection temperature approach. Other related research is reported by: Park et al in Polymer Eng. and Sci., Vol. 38, pp 1450, 1998; Briscoe et al. in Powder Technology 99, pp 228, 1998; Aoyama et al. in J. of Japan Light Metals, Vol. 43, pp 275, 1993; and Wang et al. in J. of Computer Appl. In Technology, Vol. 9, pp 211, 1996.
Consequently, need still exists for an innovation which will is overcome the drawbacks of the prior art approaches and provide an effective approach to prediction of optimal molding cycle time.
In a preferred embodiment of the invention, an optimal injection molding cycle time prediction method, designed to satisfy the aforementioned need, is provided. In this method, the injection molding cycle time is defined as the time duration from the end of the injection phase to initiation of the part ejection phase, since a large portion (more than 75%) of the cycle time is composed of the packing and cooling phase. The injection time, mainly determined by the part size and machine capacity, represents only a small portion (less than 10%) of the total cycle time. Consequently, only the packing and cooling phase is considered and the temperature at the end of the injection phase is assumed to be uniform over the entire part for sake of simplicity.
To overcome the drawbacks of the prior art approaches, a preferred embodiment of the invention uses the material yield stress as the criterion for ejection and provides an integrated approach which includes calculating the stress, at different cooling times, in the selected critical areas which represent the highest stress regions in the part. Since the stress at ejection is governed by the combined effects of processing conditions, material properties and part/pin design, this integrated approach links thermal analysis, shrinkage and friction calculations. Analysis is performed over a wide range of the cooling times until the entire thickness reaches the mold temperature. Finally, the predicted stress traces are compared to the yield stress of the polymer to determine (1) if the part can be ejected without damage, and (2) if (1) is true, the upper and lower limits of the cycle time based on the interception points between induced stress and yield stress curves. Thus, instead of using cooling analysis to predict cycle time, the invention uses an integrated methodology to assess the dependency of cycle time on material properties, processing conditions and part/pin design.
In a preferred embodiment of the invention, a method of predicting optimal injection molding cycle time comprises the steps of: performing a thermal analysis through a thickness of a part made using an injection mold; calculating shrinkage as a function of cooling time; calculating friction force between the part and mold; calculating ejection forces based on the coefficient of friction and the earlier calculated shrinkage amount; selecting high stress areas in the part and calculating induced stress in such areas; and comparing the induced stress with material yield stress to determine an optimal cycle time window during which the part can be ejected without being damaged.